It does seem counter-intuitive, however, this is not a simple subject as there are many things going on that need to be taken into account.

1) PHP is an interpreted language, this means that it depends on how PHP has interpreted your intentions when writing the code, just because you have more PHP instructions, doesn't mean that the computer will have to execute more code at CPU level

2) Different versions of PHP have worked on optimising certain parts of the PHP interpreter, if you optimised for one version of PHP, it may change when upgrading, a change somewhere else in the script could possibly affect the overall timings

3) Making timings of script execution is always a contentious issue, it will depend on what's running in the background - have you tried executing the scripts over a long period of time at different times of the day? Or did you just execute the scripts one after the other?

The long and the short of it is: if a script takes over a second or two to execute, then it would be worth looking at speed; this I hope would have been some advice that you have already come across during your " research about speed". Most speed issues are using too many unnecessary / convoluted loops and database calls.